The present invention relates to an information transmission system employing optical communication, and more particularly to a network with high reliability and flexibility using optical frequency selection and optical frequency conversion functions.
Recently, with the advance of coherent communication techniques, there has been proposed a network utilizing optical frequency division multiplexing (or optical wavelength division multiplexing) transmission.
Typical examples of the optical frequency or wavelength division multiplexing network are found in paper (1) xe2x80x9cIEEE Journal of Lightwave Technology, Vol. 7, No. 11, pp. 1759-1768, 1989xe2x80x9d and paper (2) xe2x80x9cProceedings of IOOC, ""90, pp. 84-95, 1990xe2x80x9d. Networks described in other papers are similar to those described in the above two papers.
A network configuration described in the paper (1) is shown in FIG. 2 of the paper and part thereof corresponding to the present invention is shown in FIG. 2 of the accompanying drawings. FIG. 2 shows a line distribution and collection system of the network shown in the paper (1). The system of FIG. 2 includes a remote node 10 having a wavelength demultiplexer 500 and a wavelength multiplexer 501 connected through optical fibers 100 and 200, respectively, to a central office and subscriber terminals 20-1xcx9cN connected through optical fibers 300-1xcx9cN to 400-1xcx9cN to the remote node. Signals having wavelength xcex11 to xcex1n transmitted from the central office in wavelength division multiplexing fashion are demultiplexed into signals having the respective optical frequencies by the wavelength demultiplexer to be transmitted to the subscriber terminals 20-1xcx9cN. On the countrary, signals having wavelength xcex21 to xcex2n transmitted from the subscriber terminals 20-1xcx9cN are wavelength-multiplexed by the wavelength multiplexer to be transmitted to the central office.
In the above-mentioned system, the subscriber terminals 20-1xcx9cN must transmit and receive signals having different wavelengths, respectively. In the paper (1), as shown in FIG. 4 thereof, receivers are common to the subscriber terminals, while transmitters employ lasers having different wavelengths for each subscriber terminal. Accordingly, a laser having stable wavelength must be provided in each subscriber terminal and hence there is a problem in reliability and flexibility. Further, movement of the subscriber terminal is not easy.
In the paper (1), transmission employs the conventional intensity modulation optical communication and accordingly it is difficult that the multiplex degree of optical signal exceeds 100. Even in this system, a coherent receiver capable of effecting multiplexing with the multiplex degree of 1000 or more can be used. In this case, receivers capable of receiving signals having wavelengths xcex11 to xcex1n transmitted from the central office assigned to the subscriber terminals 20-1xcx9cN with wavelength division multiplexing are required. Accordingly, the receivers are expensive as compared with the present invention described later.
Further, coherent receivers having variable transmission wavelength and common to the subscriber terminals 20-1xcx9cN can be employed. In this case, however, signals having wavelength xcex21 to xcex2n transmitted from the subscriber terminals are also multiplexed and accordingly the wavelength must be stable. It is difficult to remotely control the wavelength and hence the reliability of the network is also degraded.
Furthermore, when it is to be attempted that the optical fibers 300-1xcx9cN and 400-1xcx9cN are combined to effect bi-directional transmission by means of a single optical fiber per subscriber terminal, xe2x80x9cit is basically required that all of wavelengths xcex11 to xcex1n and xcex21 to xcex2n are differentxe2x80x9d and utilization efficiency of frequency is deteriorated.
A network configuration described in the paper (2) is shown in FIG. 1 of the paper and is shown in FIG. 3 of the accompanying drawings. in corresponding manner to the present invention. The system includes a remote node (not shown in the paper (2)) having a power divider 502 and a transport star coupler or wavelength multiplexer 501 connected to a central office (not shown in the paper (2)) through optical fibers 100 and 200 and fixed wavelength receivers and tunable transmitters or subscriber terminals 20-1xcx9cN connected to the remote node through optical fibers 300-1xcx9cN and 400-1xcx9cN. All optical signals having wavelengths xcex11 to xcex1n transmitted from the central office with wavelength division multiplexing are transmitted to the subscriber terminals 20-1xcx9cN by means of the power divider and the subscriber terminals 20-1xcx9cN receive only necessary signals by receivers for receiving only particular wavelength. On the contrary, signal-having wavelength; xcex21 to xcex2n transmitted from the subscriber terminals are wavelength-multiplexed by the wavelength multiplexer to be transmitted to the central office.
This system is featured in that an inexpensive power divider is used instead of the wavelength demultiplexer of the paper (1) and wavelength selection reception which is a maximum advantage of coherent transmission can be utilized.
The maximum drawback of this system is that all of the subscriber terminals 20-1xcx9cN can receive all signals. Thus, there is a problem in privacy characteristic.
Accordingly, in the system of the paper (2), receivers having fixed receive frequency are disposed in each of the subscriber terminals 20-1xcx9cN. However, there remains the problem in the privacy characteristic for malicious operation.
Further, when coherent transmitter and receiver are used, the transmitter and receiver of the system have also the same problem as in the transmitter and receiver of the paper (1).
The conventional network utilizing the wavelength division multiplexing has drawbacks as follows. Particularly, since the wavelength employed between the central office and the remote node and between the remote node and the subscriber terminals is the same, a failure occurring in one subscriber terminal influences all of the subscriber terminals connected to the remote node to which the subscriber terminal having the failure is connected. Further, since the transmitter and receiver of the subscriber terminal must deal with a multiplicity of frequencies and require the same reliability as that of the central office, it is very expensive. In addition, expansion of the network and rearrangement of the subscriber terminals are not made easily and the flexibility of the network is lacking.
It is an object of the present invention to provide a network having transmitters and receivers for terminals utilizing inexpensive common optical frequency division multiplexing and having good privacy characteristic, high reliability and flexibility.
In order to achieve the above object, the present invention has the following measures.
1. A node for distributing signals transmitted in optical frequency division multiplexing to terminals selects an optical frequency corresponding to the terminal from the transmitted signals and converts the selected optical frequency into an optical frequency common to the terminals as determined in an interface so as to be transmitted to the terminals.
2. A node for collecting signals transmitted from the terminals and transmitting the signals in optical frequency division multiplexing fashion converts the signals transmitted with the optical frequency determined in the interface common to the terminals into optical frequencies to be transmitted in the optical frequency division multiplexing fashion.
FIG. 1 shows a basic logical configuration of the present invention. It comprises a remote node 10 connected through optical speech paths or optical channels 100 and 200 to an upper node and terminals 20-1xcx9cN connected to the remote node 10 through optical fibers 300-1xcx9cN and 400-1xcx9cN. The remote node 10 includes optical frequency selectors 600-1xcx9cN for selecting optical frequencies in accordance with control signals 650-1xcx9cN, optical frequency converters 601-1xcx9cN for converting optical frequency in accordance with the control signals 650-1xcx9cN, optical frequency converters 602-1xcx9cN for converting optical frequency in accordance with control signals 660-1xcx9cN, and a control unit 11 for producing the control signals 650-1xcx9cN and 660-1xcx9cN. The optical frequency selectors 600-1xcx9cN select signals having optical frequencies xcex11 to xcex1n corresponding to the terminals from signals having optical frequencies xcex11 to xcex1n transmitted from the upper node through the optical channel 100 in the optical frequency division multiplexing in accordance with the control signals 650-1xcx9cN produced by the control unit 11 and the selected signals are converted into signals having optical frequency xcex10 determined in an interface common to the terminal by the optical frequency converters 601-1xcx9cN in accordance with the control signals 650-1xcx9cN of the control unit 11 to transmit the converted signals to the terminals 20-1xcx9cN through the optical fibers 300-1xcx9cN. On the contrary, signals transmitted from the terminals 20-1xcx9cN through the optical fibers 300-1xcx9cN and having optical frequency xcex20 determined in the interface common to the terminals are converted by the optical frequency converters 602-1xcx9cN into signals having optical frequencies xcex21 to xcex2n in accordance with the control signals 660-1xcx9cN of the control unit 11 and are optical frequency division multiplexed to be transmitted to the upper node.
FIG. 1 shows the logical configuration, while even if the optical frequency selection and the optical frequency conversion are replaced with each other, it can be configured by a functioning portion which performs the optical frequency selection and the optical frequency conversion simultaneously.
Further, the optical frequency of the signals between the terminals and the node is not limited to one kind, and a system in which the optical frequency is selected from predetermined frequencies can be configured.
Transmission between the terminals and the node can be made by the optical frequency division multiplexing transmission and further by the optical frequency division multiplexing bi-directional transmission. At this time, a plurality of optical frequencies between the terminals and the node common to the terminals are required.
According to the present invention, since the signal having the frequency corresponding to the terminal is selected by the optical frequency selector and only the signal is optical frequency division multiplexed to be transmitted to the terminal, the privacy is ensured.
Further, since the optical frequencies for communication between the upper node and the remote node and between the remote node and the terminals are assigned independently and are controlled by the control unit of the remote node, the reliability is high and the flexibility is increased. In addition, by assigning the optical frequencies between the upper modes and the remote node dynamically, a highly reliable and flexible network can be realized.
The transmit and receive optical frequency of the terminal is common to the terminals and fixed, and the frequency range is narrow. Even when a plurality of optical frequency are assigned, frequency spacing may be made wide and accordingly inexpensive and reliable terminals can be attained.